JPH10308028A - Optical system for optical head - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an optical system for an optical head which can perform reading and writing for a plurality of optical disks having different protection substrate thickness. SOLUTION: A ring band opening 21 is arranged on an optical axis of an object lens 10 of an optical system for an optical head for reading and writing a signal from/in an optical disk 31. Light can be transmitted through only a peripheral part of an object lens 10 by the ring band opening 21. The numerical aperture of the inside diameter opening and the numerical aperture of the outside opening of the ring band opening 21 are decided so that aberration caused in the object lens 10 by difference of thickness and a refraction index of a protection substrate 31a by difference of kinds of the optical disk 31 performing reading and writing is stored in an aberration range in which data can be read and written by a laser diode 40, a light receiving element 70, and the like.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an optical system for an optical head capable of reading from or writing to a plurality of types of optical disks.

[0002]

2. Description of the Related Art Optical disks used for reading or writing signals include a magneto-optical disk (MO disk), a phase modulation disk (CD), and a phase change disk (PD) according to the principle of signal reading. are categorized. Each of these optical disks has a thickness of 1.2
The signal surface inside the optical disk is protected by the protective substrate of mm.

On the other hand, a digital video disc (DVD) developed to increase the recording density of information is 0.6
The signal surface is protected by a protective substrate of mm. Therefore, when trying to read a record from a disk having a protective substrate thickness of 0.6 mm with an optical head designed for a protective substrate thickness of 1.2 mm, the optical system of the optical head is caused by a difference in the protective substrate thickness. Causes extremely large spherical aberration (wavefront aberration), and it is not possible to narrow down the focused spot sufficiently.
This will be described with reference to FIG.

FIG. 6 is a diagram for explaining the state of focusing of laser light on the signal surface of an optical disk. In FIG. 6A, the laser beam transmitted through the circular aperture 20 is focused on the signal surface 30b of the optical disk 30 by the objective lens 10 whose aberration is well corrected for an optical disk having a substrate thickness of 1.2 mm. Show the situation. However, as shown in FIG. 6B, the signal surface 31b of the optical disc 31 having a substrate thickness of 0.6 mm is used.
, A spherical aberration occurs and the signal cannot be read.

Several proposals have been made to solve the above-mentioned problems that occur when data is read from optical disks having different protective substrate thicknesses with one device.

Here, some schemes already proposed will be described. In the head switching method, dedicated optical heads respectively corresponding to optical disks having different protective substrate thicknesses are built in the device, and these dedicated heads are switched and used according to the protective substrate thickness of the optical disk from which data is read. In the optical system switching system, the main parts such as the light emitting and receiving part and the driving part of the head are shared, and two kinds of lenses corresponding to the protective substrate thickness of the optical disk are incorporated in this head so that they can be switched by a turret type switching device etc. Things.

[0007] A hologram system or a stop-down system in which the main part of the head and the lens are shared has been proposed in addition to the above-mentioned switching system. In the hologram method, a hologram is formed or separately provided on a part of the lens surface of an optical head, and light emitted from a light source such as a laser diode (LD) is transmitted through the hologram part of the lens and diffracted and condensed. The light is divided into light transmitted through a portion other than the hologram and collected. Then, the divided light can be focused on disks having different protective substrate thicknesses. In the stop-down method, a variable stop is provided adjacent to the lens of the optical head, and the variable stop is stopped in order to reduce spherical aberration caused by a difference in the substrate thickness of the disk.

[0008]

However, each of the above-mentioned methods has the following problems. That is, in the head switching method, a plurality of dedicated pickups corresponding to the respective substrate thicknesses must be prepared, resulting in an increase in the size of the apparatus and an increase in cost. The optical switching method requires a lens switching mechanism, which leads to a decrease in reliability and an increase in cost due to the complexity of the optical head. According to the hologram method, very advanced mold processing technology and glass forming technology were required to form a hologram on a head lens. In the aperture stop method, for example, by using a transmission type liquid crystal shutter as a variable stop, there is no movable part, and therefore, an optical head having a simple structure and excellent reliability can be realized. There has been a problem of a decrease in light quantity and a decrease in resolution.

An object of the present invention is to provide an inexpensive and highly reliable optical system capable of reading and writing information from and to various optical disks having different protective substrate thicknesses.

[0010]

FIG. 3 shows an embodiment of the present invention.
The present invention will be described with reference to FIG. (1) The objective lens 10 guides the light emitted from the light source 40 to the signal surface 31 b of the optical disk 31 and guides the light reflected by the signal surface of the optical disk 31 to the light receiving element 70. An annular opening 21 provided on the optical axis of the objective lens 10 for limiting the opening of the objective lens 10;
In addition, the numerical aperture of the inner diameter and the numerical aperture of the outer diameter are determined so that the aberration caused by the difference in the protective substrate thickness of the second optical disk or the difference in the refractive index of the protective substrate falls within the aberration range in which data can be read and written. Achieves the above object. (2) The invention described in claim 2 satisfies the following conditional expressions. G (NA1, NA2) × | T1 · n c 1-T2 · n c 2 |
/Λ<0.21 where G (NA1, NA2) = ｛(1−NA2 / NA1) ² +
0.3 × (1-NA2 / NA1)｝ × 650 × (NA1
/0.60) ⁴ × 1 / 1.5 λ: wavelength of the light source 40 used in the optical head T1: protective substrate thickness of the first optical disk T2: protective substrate thickness of the second optical disk (T1> T)
2) n _c 1: refractive index of the protective substrate of the first optical disk n _c 2: refractive index of the protective substrate of the second optical disk NA1: inner aperture of the annular aperture 21 NA2: outside the numerical aperture of the annular aperture 21 (3) In the invention according to claim 3, the first and second optical discs comply with the following conditional expression (1), and the orbital opening 21 complies with the following conditional expression (2). The conditional expression (1): | T1 · n c 1-T2 · n c 2 | / 1.
5 <0.7 (mm) Conditional expression (2): NA2 / NA1> 0.5 where T1: protective substrate thickness of the first optical disk (mm) T2: protective substrate thickness of the second optical disk (mm, T
1> T2) n _c 1: refractive index of the protective substrate of the first optical disk n _c 2: a second refractive index of the protective substrate of the optical disk NA1: inner aperture of the annular aperture NA2: outside the numerical aperture of the annular aperture

In the section of the means for solving the above-mentioned problems, which explains the configuration of the present invention, the drawings of the embodiments of the present invention are used to make the present invention easier to understand. However, the present invention is not limited to this.

[0012]

FIG. 1 shows an embodiment of the present invention.
This will be described with reference to FIGS.

Occurs when trying to write or read signals to or from an optical disk with a protective substrate thickness of 0.6 mm in an optical system for an optical head whose aberration has been corrected for an optical disk with a protective substrate thickness of 1.2 mm. FIG. 1 shows the spherical aberration. In the graph of FIG. 1, the horizontal axis represents the height (distance from the optical axis) of the light beam entering the objective lens 10, and the vertical axis represents the light beam entering the objective lens 10 at each height. Represents the position intersecting the optical axis. In this spherical aberration diagram, the height of the light beam is NA1 (corresponding to the radius of the circular aperture 20).
At this time, the spherical aberration has a peak value W0. Here, in consideration of a case where a ring-shaped opening having an inner diameter NA2 and an outer diameter NA1 is inserted instead of the circular opening 20, the spherical aberration decreases from WI to W0 in the graph of FIG. As described above, by inserting the annular opening instead of the circular opening, FIG.
As shown in (2), the spherical aberration can be greatly reduced, and the imaging performance can be improved.

Here, a CD is assumed as an optical disk having a protective substrate thickness of 1.2 mm, and a DVD is assumed as an optical disk having a protective substrate thickness of 0.6 mm.
In an optical system for an optical head in which aberration is corrected for an optical disk having a protective substrate thickness of 2 mm, the spherical aberration generated when trying to read a signal from an optical disk having a protective substrate thickness of 0.6 mm and the annular aperture are reduced. A description will be given of the effect of reducing spherical aberration by using. The meanings of the symbols used in the description are as follows. λ: wavelength used NA0: numerical aperture determined by the diameter of the circular aperture 20. NA1: a numerical aperture determined by the outer diameter of the annular opening 21. NA2: The numerical aperture determined by the inner diameter of the annular opening 21. T1: CD protective substrate thickness. T2: DVD protective substrate thickness. nc: refractive index of the protective substrate. ρ: coordinates on the pupil.

Since a higher resolution is required for DVD, NA1 is larger than NA0 (numerical aperture of a circular opening for CD).
(The outer diameter NA of the orbicular zone opening for DVD) may be increased, but even when accessing a CD, the NA may be increased.
Since the larger the value of, the narrower the condensing spot, the NA0 (the numerical aperture of the circular aperture for CD) is set to N
The following description will be made on the assumption that NA1 = NA0 without making the diameter smaller than A1 (outer diameter NA of the annular zone opening for DVD).

Here, assuming that the aberration of the objective lens 10 is corrected for a CD, that is, for an optical disk having a protective substrate thickness of 1.2 mm, a D-axis having a protective substrate thickness of 0.6 mm is used.
A large spherical aberration occurs for VD. When discussed in the third order aberration theory, it is known that the magnitude of this spherical aberration is proportional to the amount of change in the protective substrate thickness (T2−T1) and is given by the following equation (optical disc technology, supervised by Morio Onoe) Co-authored by Murayama, Koide, Yamada, Kunikane, '89 Radio Technology, pp. 61).

W (ρ) = W0 × (ρ / NA1) ⁴ Expression (1) W0 = | T2-T1 | × (nc ² -1) × (NA1) ⁴ / (8 × nc ³ ) Expression (2) ρ: 0 to NA1 (incident height of light beam normalized by NA)

Equation (1) is a spherical aberration on the Gaussian image plane, not a spherical aberration on the best image plane. In the optical system for an optical head, the tracking of the best image plane is automatically performed by the autofocus mechanism. Therefore, the meaning is not the spherical aberration on the Gaussian image plane but the spherical aberration on the best image plane. Then, since the defocus component is removed by autofocus so as to minimize the RMS value of the wavefront aberration (hereinafter, RMS wavefront aberration), the spherical aberration on the best image plane is generally reduced on the Gaussian image plane. Is smaller than the aberration at. In the above equation (2), W0
Means the peak value of spherical aberration on the Gaussian image plane.

Here, the best image plane is a distance Z from the Gaussian image plane.
In this case, the spherical aberration at the best image plane is expressed by the following equation (3).

[Number 2] W (ρ) = W0 × ( ρ / NA1) 4 -Z × (ρ / NA1) 2/2 ... Equation (3) formula (3), the distance Z is RMS wavefront aberration as described above It can be determined from the condition of minimizing. In the optical system for an optical head, as described above, the best image plane is automatically tracked by the autofocus mechanism, and therefore, what value Z takes is not important.

It is known that the following relationship is established between W0 and the RMS wavefront aberration ΔW at the best image plane (Principles of Optics, 6-th Ed., M. Born, EW).
olf co-authored, '93 Pergamonn Press, USA, p.
p. 464).

ΔW = W0 / (6 × 5 ^1/2 ) Equation (4)

By applying specific numerical values to the above equation, an optical system for an optical head in which aberration has been corrected for a CD having a substrate thickness of 1.2 mm can be used for a DVD having a substrate thickness of 0.6 mm. The following describes how much aberration occurs. As an example, λ = 680 nm
m, NA1 = 0.6, T1 = 1.2 mm, T2 = 0.6
When the calculation is performed in the case of mm and nc = 1.5, the spherical aberration caused by the protection substrate thickness being changed from 1.2 mm to 0.6 mm is obtained by substituting each value into the equation (2) and obtaining W0 = 5.3
λ. Therefore, the RMS wavefront aberration ΔW at the best image plane is ΔW = 0.39λRMS, which is a very large value according to equation (4).

T1 = 0.9 mm, T2 = 1.2 m
Even when calculation is performed in the case of m or 0.6 mm (other conditions are the same as the above conditions), ΔW = 0.20λRMS, which is a very large value.

Normally, the standard of wavefront aberration that can be practically used in an optical system for an optical head used for reading and writing on an optical disk is λ / 1.
4RMS (≒ 0.07λRMS) or less, which far exceeds this criterion in the above-described example, and it is impossible to read a signal from a DVD.

The above-described spherical aberration can be reduced by inserting the annular opening 21 into the objective lens 10 (see FIG. 2). At this time, the position of the best image plane also moves from the best image plane position at the time of the circular aperture, but there is no problem because it is corrected by the autofocus mechanism as described above.
Hereinafter, this example will be described.

The spherical aberration and the best image plane position when the annular aperture 21 is inserted into the objective lens 10 can be obtained in the same manner as when the circular aperture 20 is inserted. That is, based on the spherical aberration calculated by the equation (3), R
If the MS wavefront aberration ΔW is determined and Z is determined such that the RMS wavefront aberration ΔW is minimized, the Z becomes the best image plane position when the annular aperture is inserted. On the other hand, the RMS wavefront aberration ΔW is
It can be obtained by integrating and averaging the spherical aberration W (ρ) of the light beam transmitted through the annular aperture. That is, the RMS wavefront aberration ΔW is calculated by the following equations (5) to (7), and Z that minimizes this ΔW may be obtained. This Z is the best image plane position when the annular opening 21 is inserted, and the spherical aberration at that time becomes the spherical aberration at the best image plane position.

(Equation 4)

By the way, in a read-only optical system for reading a CD or DVD, unlike an optical system for a rewritable disk, there is no need to erase or record information.
The required amount of light energy is also small. Therefore, as described above, the light utilization efficiency (the amount of light passing through the circular
The decrease in the ratio of the amount of light passing through the annular opening 21) causes almost no problem, but the light use efficiency η can be appropriately estimated by the following equation (8).

(Equation 5) Where K = φobj / φbeam (number of beams), φobj: aperture diameter of the objective lens (aperture size) φbeam: beam size of Gaussian beam (peripheral intensity is 1 / e ² with respect to beam center intensity) Beam diameter when it becomes

Next, the main part of the optical system for an optical head according to the embodiment of the present invention will be described with reference to FIG. FIG.
The optical system for an optical head includes a laser diode (LD) 40 for emitting a laser beam, a collimator lens 50 for converting the laser beam into parallel light, and an annular aperture 21.
And an objective lens 10 for converging the parallel light guided by the collimator lens 50 on the signal surface (signal surface 31b in FIG. 3) of the optical disk, and reflected by the signal surface 31b and guided by the condensing lens 10 and the collimator lens 50. A beam splitter 60 for guiding the emitted laser light downward in the figure and a light receiving element 70 for converting the laser light guided by the beam splitter 60 into an electric signal are provided.

The optical system for an optical head shown in FIG. 3 further includes a focus detection circuit 80 for detecting a focus position of the laser beam guided to the light receiving element 70, and a focusing actuator based on a focus detection signal from the focus detection circuit 80. A control circuit 81 for issuing a control signal to the control circuit 82 and a focusing actuator 82 for driving the holding frame 83 in the optical axis direction of the objective lens 10 based on the control signal from the control circuit 81 are provided.

With the above configuration, in the optical system for an optical head shown in FIG. 3, the light is emitted from the LD 40, guided to the signal surface 31b of the optical disk through the collimator lens 50, the annular opening 21, and the objective lens 10, and Signal plane 31
The b-reflected laser light is focused on the light receiving element 70. The annular aperture 21 satisfactorily corrects aberrations irrespective of the difference in the thickness or the refractive index of the protective substrate of the optical disk from which signals are read or written.

[0029]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, three examples of an optical head optical system according to an embodiment of the present invention will be described.

(Embodiment 1) Various conditions in an optical system for an optical head are determined as follows.

[Table 1] Wavelength: λ = 650 nm Refractive index of protective substrate: nc = 1.5 Number of pieces: K = 0.9 DVD protective substrate thickness: T1 = 0.6 mm CD protective substrate thickness: T2 = 1.2 mm Objective Lens aberration correction target: protective substrate thickness 0.6 m
m (DVD). Condition of annular opening Numerical aperture (NA): outer diameter NA = 0.6 (the inner diameter NA is treated as a variable in the following description)

RMS generated when trying to read a CD signal with the optical system for an optical head under the conditions shown in Table 1 above.
The wavefront aberration ΔW will be described with reference to FIG. FIG. 4A is a graph in which the RMS wavefront aberration ΔW and the light use efficiency when the inner diameter NA of the annular opening is changed based on the conditions of Table 1 are calculated and plotted, and the horizontal axis indicates the annular opening. , The wavefront aberration is shown on the left side of the vertical axis, and the light use efficiency is shown on the right side of the vertical axis. Note that FIG.
In (a), the light use efficiency η is obtained by assuming that a circular aperture having an outer diameter NA = 0.6 is 100%.

As can be seen from FIG. 4A, the wavefront aberration ΔW generated in the optical system for the optical head under the conditions shown in Table 1.
When the inner diameter NA = 0.48, the wavefront aberration ΔW = 0.052
λRMS When the inner diameter NA = 0.53, the wavefront aberration ΔW = 0.016
λRMS The light use efficiency η at this time is: light use efficiency η = 20% when inner diameter NA = 0.48, light use efficiency η = 10% when inner diameter NA = 0.52, and there is no practical problem. Value.

By the same calculation, the inner diameter NA of the annular opening when the wavefront aberration becomes 0.07λRMS, which is the resolution limit of the optical system for an optical head, is obtained as NA = 0.46. When the inner diameter NA at the time of the resolution limit is normalized by the outer diameter NA, 0.46 / 0.6 = 0.77. That is,
The conditions under which the correction of aberration by the annular aperture is effective are:
When expressed in general using the outer diameter NA and the inner diameter NA,
Inner diameter NA / outer diameter NA> 0.77.

On the other hand, in the optical system for an optical head described above, if a circular opening having an outer diameter NA = 0.6 is inserted instead of the annular opening, a CD (protective substrate thickness = 1.2 m) is inserted.
The wavefront aberration generated when accessing m) is W0 = 5.54λΔW = 0.41λRMS from Expressions (2) and (4), and the aberration increases drastically. As described above, in a normal optical head optical system, the value of .DELTA.W = 0.07.lambda.
41λRMS is a considerably large value, and it can be said that no resolution is obtained at all. That is, it can be seen that the annular aperture of the optical system for an optical head according to Example 1 effectively corrects aberration. (Embodiment 2) The conditions are as follows: the aberration of the optical system for an optical head is CD
Table 1 (that is, when reading a DVD), except for the point of 0.9 mm, which is the average of the protective substrate thickness (1.2 mm) and the DVD protective substrate thickness (0.6 mm). Also, when reading a CD, the aberration is distributed while the aberration remains.)

The RMS wavefront aberration .DELTA.W generated when trying to read a CD signal with the optical system having an annular aperture under this condition will be described with reference to FIG. 4B. FIG. 4B shows the RM when the inner diameter NA of the orbicular zone opening is changed based on the conditions of the second embodiment described above.
5 is a graph in which S wavefront aberration and light use efficiency are calculated and plotted. The horizontal and vertical axes of the graph are shown in FIG.
Is the same as that of the graph shown in FIG.

As can be seen from FIG. 4B, the aberrations are as follows due to the effect of the annular aperture. When the inner diameter NA = 0.48, the wavefront aberration ΔW = 0.028
λRMS When the inner diameter NA = 0.53, the wavefront aberration ΔW = 0.09
λRMS The light utilization efficiency at this time is as follows: when the inner diameter NA = 0.48, the light utilization efficiency η = 20%, when the inner diameter NA = 0.53, the light utilization efficiency η = 10%. This is a value that does not cause any problem.

Subsequently, when the inner diameter NA of the orbicular zone opening when the wavefront aberration becomes 0.07λRMS which is the resolution limit of the optical system for an optical head is obtained from FIG. 4B, NA = 0.39. . When the inner diameter NA at the time of the resolution limit is standardized by the outer diameter NA, 0.39 / 0.6 = 0.65. That is, the object of aberration correction of the objective lens is set to a protective substrate thickness of 0.9 mm.
In this case, when the condition under which the correction of aberration by the annular aperture is effective is expressed using the outer diameter NA and the inner diameter NA, it is expressed as inner diameter NA / outer diameter NA> 0.65.

On the other hand, a circular aperture having an outer diameter of NA = 0.6 is inserted into the optical system for an optical head described above, and the DVD or C is inserted.
The wavefront aberration generated when D is reproduced is expressed as follows: W0 = 2.77λΔW = 0.21λRMS by substituting 0.3 into the term | T2−T1 | in equation (2). Even in this case, ΔW = 0.07λ which is an allowable value of aberration
The value is still large for the RMS, and the effect of reducing the aberration by the annular aperture can also be confirmed here.

(Embodiment 3) Prior to the description of Embodiment 3, D
Pits formed on the signal surface of VD or CD will be described. The size of a pit formed on the signal surface of a DVD is smaller than that of a pit formed on the signal surface of a CD in order to increase the recording density. Accordingly, a higher resolution is required for an optical head optical system for reading a DVD signal. More specifically, the shortest pit length formed on each optical disk is 0.4 μm for a DVD and 0.9 μm for a CD.
When a signal is read from a DVD, a resolution approximately twice that of a CD is required. In other words, CD
The resolution required for reading may be about half as low as the resolution required for reading DVD. Therefore, the wavelength of the light source used for the optical head is CD, DV
D When reading any of the optical disks, if the light has the same wavelength (650 nm in this embodiment), the CD
The wavefront aberration generated in the optical system for an optical head at the time of reading may be larger than 0.07λRMS.

Here, the level of the wavefront aberration at which the CD can be read will be described with reference to FIG. FIG. 5 (a) calculates and plots the MTF value that changes according to the amount of RMS wavefront aberration generated by the objective lens 10 (FIG. 3) of the optical system for the optical head with respect to the minimum pit length of 0.9 μm. It is a graph. The parameters used for the calculation are λ = 650 nm and NA = 0.60. The MTF level at which the minimum pit length (0.9 μm in the example of FIG. 5A) can be resolved is generally 25% or more, and the wavefront aberration is 0.14 λRM from the graph of FIG.
It can be said that the smallest pits formed on the signal surface of the CD can be read even if about S exists. In other words, it can be said that when a signal is read from a CD, there is about twice as much wavefront aberration as when a signal is read from a DVD.

Therefore, in the second embodiment, the aberration correction target of the objective lens of the optical system for an optical head is set to a protective substrate thickness (D) of 0.9 mm.
In the third embodiment, the thickness of the protection substrate for aberration correction should be closer to the thickness of the DVD protection substrate than 0.9 mm, while the protection substrate thickness of the VD and CD is 0.6 mm and the average value of 1.2 mm. Thus, more efficient distribution of aberrations can be performed. That is, the wavefront aberration levels at which the optical discs of DVD and CD can be read are 0.07λRMS, respectively.
Since it is 0.14λRMS, the protective substrate thickness of the DVD is 0.
6 mm and CD protective substrate thickness of 1.2 mm are 0.07:
0.8 mm, which is a thickness internally divided into 0.14, may be set as the thickness of the protection substrate to be aberration-corrected. By determining the aberration correction target in this manner, the MTF levels at which the DVD and CD optical disks can be read can be set in a well-balanced manner.

As described above, the target of aberration correction of the objective lens of the optical system for an optical head is set to 0.8 mm in protective substrate thickness, and D
The wavefront aberration generated when reading a signal from the VD will be described with reference to FIG. FIG. 5B is a graph in which the RMS wavefront aberration and the light use efficiency when the inner diameter NA of the annular opening is changed are calculated and plotted. The parameters used in this calculation are the same as those shown in Table 1 of Example 1 except that the thickness of the protection substrate to be corrected is set to 0.8 mm. In FIG. 5B, the horizontal axis and the vertical axis of the graph are the same as those of the graph shown in FIG. 4A, and a description thereof will be omitted.

When the wavefront aberration is 0.07λRMS, which is the resolution limit of the optical system for an optical head, the inner diameter N of the annular opening
When A is read from FIG. 5B, the inner diameter NA becomes 0.32. When this inner diameter NA at the time of the resolution limit is normalized by the outer diameter NA, 0.32 / 0.6 = 0.53. That is, the aberration correction target of the objective lens is set to the protective substrate thickness 0.8 mm.
In this case, the condition under which the correction of aberration by the annular aperture is effective is expressed by using the outer diameter NA and the inner diameter NA, and the inner diameter NA / outer diameter NA> 0.53. 5B, the light use efficiency η when the inner diameter NA is 0.32 is a very high value of 54%. Incidentally, the annular zone opening (outer diameter NA = 0.
6, inner diameter NA = 0.32) and a circular opening (NA = 0.
6) is about 3: 4. In the calculation of the light use efficiency η, it was assumed that the number of beams was 0.9,
If the central part of the Gaussian beam (the part of the light source where the drop in illuminance is small) is used, the light use efficiency η is further improved, and
It can also be expected to increase to about 0% or more.

Based on the three embodiments described above,
The above-mentioned wavefront aberration is generally treated. Here, description will be given with reference numerals defined as shown in Table 2 below.

[Table 2] Inner diameter NA of annular opening: NA1 Outer diameter NA of annular opening: NA2 Wavelength of light source of optical head optical system: λ Change amount of protective substrate of optical disk: ΔT (Protection of optical disk for reading signal) The difference between the substrate thickness and the thickness of the protection substrate that has been subjected to aberration correction.
MS wavefront aberration: ΔW

For convenience, the following description assumes that ΔT = 0
In this case, the aberration remaining in the optical system for an optical head is set to zero.

The RMS aberration correction curve shown in FIG.
When fitting is performed with a quadratic curve using NA1 and NA2 as parameters, the following equation is obtained.

ΔW (λRMS) = 0.60 × (1−NA2 / NA1) ² + 0.18 × (1−NA1 / NA2) Equation (9)

Equation (9) shows that when the value of NA1 approaches the value of NA2, that is, when the width of the annular opening approaches 0, the wavefront aberration also approaches 0. Equation (9) is λ =
650 nm, ΔT = 0.6 mm,
In general, the wavefront aberration is the variation ΔT of the protective substrate thickness of the optical disk.
And NA ⁴ are proportional to the wavelength and inversely proportional to the wavelength.
It can be normalized by T and λ, and is expressed as follows.

ΔW = {0.60 × (1-NA2 / NA1) ² + 0.18 × (1-NA2 / NA1)} × (650 / λ) × (ΔT / 0.6) × (NA1 / 0 .6) ⁴ = {(1-NA2 / NA1) ² + 0.3 × (1-NA2 / NA1)} × (650 / λ) × ΔT × (NA1 / 0.6) ⁴ Equation (10) Optical head The condition under which a signal can be sufficiently read by the optical system for practical use is expressed by the following equation (11).
0), a part of the formula is rearranged and expressed as formula (1)
2) is obtained.

ΔW <0.07λRMS Expression (11) G (NA1, NA2) × ΔT / λ <0.07 Expression (12) where, G (NA1, NA2) = ｛(1−NA2 / NA1) ) ² + 0.3 × (1-NA2 / NA1)｝ × 650 × (NA1 / 0.60) ⁴ × (650 / λ) × ΔT × (NA1 / 0.6) ⁴ Equation (13)

As a premise of the above description, it is assumed that the aberration remaining in the optical system for an optical head when ΔT = 0 is zero.
The conditional expression when the residual aberration is not 0 will be described. If the amount of aberration remaining in the optical system for an optical head when ΔT = 0 is W, it occurs due to the variation (ΔT) in the thickness of the protective substrate. The sum of the squares of the wavefront aberration ΔW and the residual aberration W is 0.
It does not need to exceed 07λRMS. Therefore, at this time, it can be expressed by Expression (14) instead of Expression (12).

G (NA1, NA2) × ΔT / λ <(0.07 ² −W ² ) ^1/2 Equation (14)

The above is the condition that the same optical head optical system can read signals from optical disks having different protective substrate thicknesses by using the annular aperture. The following conclusions can be obtained by organizing how to determine the aberration correction target.

When an annular aperture is inserted in the objective lens of the optical system for the optical head whose aberration has been corrected with respect to the optical disk having the protective substrate thickness T1, a signal from the optical disk having the protective substrate thickness T2 (T1 ≠ T2) is generated. The condition that can be read is given by the formula (1)
5).

G (NA1, NA2) × | T1−T2 | / λ <0.07 (15)

When the protective substrate thickness of the optical disk having a plurality of readable protective substrate thicknesses is represented by T1, T2 (T1 ≠ T2), the optical head for which the aberration is corrected with respect to the protective substrate thickness (T1 + T2) / 2. When the annular aperture is inserted in the objective lens of the optical system, the condition under which signals can be read from both the optical disks having the protective substrate thicknesses T1 and T2 is that the signals be read from the optical disk having the protective substrate thickness T1. In the case where a signal is read from the optical disk having the protection substrate thickness T2, ΔT = | T1-
T2 | / 2, so it is expressed as in equation (16).

G (NA1, NA2) × | T1-T2 | / λ <0.14 Expression (16)

When the protective substrate thickness of an optical disk having a plurality of readable protective substrate thicknesses is represented by T1 and T2 (T1 ≠ T2), protection that internally divides the thickness difference between the protective substrate thicknesses T1 and T2 into 1: 2. When the annular opening is inserted in the objective lens of the optical system for the optical head, the aberration of which has been corrected for the substrate thickness,
The condition under which a signal can be read from the optical disk having the protective substrate thickness T1 is as follows in Expression (12): ΔT = | T1-T2 | /
3, so it is expressed as in equation (17).

G (NA1, NA2) × | T1−T2 | / λ <0.21 Expression (17) At this time, the condition under which a signal can be read from the optical disk having the protection substrate thickness T2 is as follows in Expression (12). ΔT = | T1-
T2 | / (3/2), and the minimum pit length formed on the optical disk having the protective substrate thickness T2 is equal to the protective substrate thickness T
Since the length of the shortest pit formed on one optical disc is about twice as long, the readable condition is relaxed to twice, and is expressed as in equation (19). Then, this equation (1)
Rearranging 8) gives the same as equation (17).

G (NA1, NA2) × | T1−T2 | /1.5/λ <(0.07 × 2) Equation (18)

As described above, in order to enable signals to be read from two types of optical disks having different protective substrate thicknesses by the same optical head optical system, the resolving power required when reading signals from each optical disk is required. The thickness of the protective substrate to be corrected for the aberration of the objective lens may be set according to. At this time, when reading signals from two types of optical discs having different protective substrate thicknesses, if the same resolution is required for optical discs having both protective substrate thicknesses, equation (16) is used.
When the resolution of the optical system for the optical head required for an optical disk having one protective substrate thickness is required to be twice the resolution of the other, the annular aperture may be set according to the equation (17).

In the above description, the refractive indices of the protective substrates of the optical disks having different protective substrate thicknesses are equal and are both 1.5. However, these may have different refractive indices. That is, the wavefront aberration generated in the optical system for an optical head is caused by the change in the optical path length due to the change in the thickness of the protective substrate. Since the optical path length may be expressed as: optical path length = protective substrate thickness × protective substrate refractive index, when symbols are defined as shown in Table 3, for example, equation (17) is expressed by the following equation (19). Can be.

[Table 3] Refractive index of protective substrate of optical disk with protective substrate thickness T1: nc1 Refractive index of protective substrate of optical disk with protective substrate thickness T2: nc2

G (NA1, NA2) × | T1 · (nc1 / 1.5) −T2 · (nc2 / 1.5) | / λ <0.21 Expression (19) Alternatively, G (NA1, NA2) ) × (1 / 1.5) is redefined as G (NA1, NA2), and the expression (1)
9) can be represented by equation (20).

G (NA1, NA2) × | T1 · nc1−T2 · nc2 | / λ <0.21 Equation (20) where G (NA1, NA2) = ｛(1−NA2 / NA1) ² + 0.3 × (1-NA2 / NA1)｝ × 650 × (NA1 / 0.60) ⁴ × 1 / 1.5 Equation (21)

In the above description, it has been assumed that two types of optical disks having different protective substrate thicknesses are both read-only, but one or both of these disks are rewritable disks, that is, one is a magneto-optical disk. Alternatively, even if the disc is a phase-change disc and the other is a read-only DVD or a rewritable DVD, the above-mentioned annular opening is effective.

Furthermore, the wavelength of the light source used in the optical system for the optical head is described as being the same when reading a signal from a DVD and when reading a signal from a CD. However, light of a different wavelength is used. You may. However, there is no problem in reading signals even if the wavelengths are the same. Rather, using the same wavelength is more economical because a single laser diode and a single objective can be used.

Although only the third-order aberration is considered in the above description of the embodiment, the same effect can be obtained even if higher-order aberrations are considered. Further spherical aberration (wavefront aberration)
In addition, even if the residual aberration is coma or astigmatism, the aberration can be reduced by arranging the annular aperture in the optical system for the optical head, and the resolving power is improved.

[0058]

As described above, according to the present invention, the annular aperture is arranged on the optical axis of the objective lens of the optical system for the optical head, and the difference in the protective substrate thickness of a plurality of different optical disks or the difference in the protective substrate. Different protections for the same optical head optical system by determining the inner diameter NA and outer diameter NA so that aberrations caused by the difference in refractive index in the optical system for the optical head fall within the aberration range where data can be read or written. Writing or reading can be performed on an optical disk having a substrate thickness. In addition, the outer diameter N of the annular aperture is smaller than the case where spherical aberration is corrected by simply reducing the aperture.
Since A can be set to be larger, the condensing spot can be narrowed, and thus an optical system for an optical head having excellent resolution can be obtained.

[Brief description of the drawings]

FIG. 1 attempts to write or read data on an optical disk having a protective substrate thickness of 0.6 mm with an optical system for an optical head that has been aberration-corrected on the assumption that writing or reading is performed on an optical disk having a protective substrate thickness of 1.2 mm. 7 is a graph showing spherical aberration that occurs when performing the following.

FIG. 2 is a longitudinal sectional view showing a state in which aberration is corrected by an objective lens of an optical system for an optical head according to the present invention, and a light beam forms an image on a signal surface of an optical disk.

FIG. 3 is a diagram showing a main part of an optical system for an optical head according to an embodiment of the present invention.

FIG. 4 is a graph showing changes in wavefront aberration and light use efficiency when the inner diameter NA of the orbicular zone is changed in the optical system for an optical head according to the embodiment of the present invention;
(A) shows a case where the change amount ΔT of the protective substrate thickness is 0.6 mm, and (b) shows a case where the change amount ΔT of the protective substrate thickness is 0.3 mm.

5A and 5B are graphs showing a change in optical performance of the optical system for an optical head according to the embodiment of the present invention. FIG. 5A shows a state in which an MTF value changes with a change in wavefront aberration, and FIG. () Shows changes in wavefront aberration and light use efficiency when the inner diameter NA of the annular opening is changed when the change amount ΔT of the protective substrate thickness is 0.2 mm.

FIG. 6 is a diagram showing a state in which light is condensed on a signal surface of an optical disk having a different protective substrate thickness by an optical system for an optical head according to a conventional technique, and FIG. (B) shows a state where aberration is generated.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Objective lens 20 Circular aperture 21 Ring aperture 30, 31 Optical disk 30a, 31a Protective substrate 30b, 31b Signal surface 40 Laser diode 50 Collimator lens 60 Beam splitter 70 Light receiving element

Claims (3)

[Claims] An objective lens that guides light emitted from a light source to a signal surface of an optical disk and guides light reflected by the signal surface of the optical disk to a light receiving element; and an objective lens provided on an optical axis of the objective lens; An annular aperture for limiting the aperture of the objective lens, wherein the annular aperture reduces aberration caused by at least a difference in the protective substrate thickness of the first and second optical discs or a difference in the refractive index of the protective substrate. An optical system for an optical head, wherein the numerical aperture of the inner diameter and the numerical aperture of the outer diameter are determined so as to fall within an aberration range in which data can be read and written. 2. The optical system for an optical head according to claim 1, wherein the following conditional expression is satisfied. G (NA1, NA2) × | T1 · n c 1-T2 · n c 2 |
/Λ<0.21 where G (NA1, NA2) = ｛(1−NA2 / NA1) ² +
0.3 × (1-NA2 / NA1)｝ × 650 × (NA1
/0.60) ⁴ × 1 / 1.5 λ: wavelength of light source used in optical head T1: protective substrate thickness of first optical disk T2: protective substrate thickness of second optical disk (T1> T)
2) n _c 1: first optical disk having a protective substrate having a refractive index n _c 2: a second refractive index of the protective substrate of the optical disk NA1: inner aperture of the annular aperture NA2: outside the numerical aperture of the annular aperture 3. The optical system for an optical head according to claim 1, wherein the first and second optical disks have the following conditional expression (1).
The optical system for an optical head is characterized in that the annular opening conforms to the following conditional expression (2). The conditional expression (1): | T1 · n c 1-T2 · n c 2 | / 1.
5 <0.7 (mm) Conditional expression (2): NA2 / NA1> 0.5 where T1: protective substrate thickness of the first optical disk (mm) T2: protective substrate thickness of the second optical disk (mm, T
1> T2) n _c 1: refractive index of the protective substrate of the first optical disk n _c 2: a second refractive index of the protective substrate of the optical disk NA1: inner aperture of the annular aperture NA2: outside the numerical aperture of the annular aperture